The invention relates to a method for operating an internal combustion engine of a hand-guided power tool, for example, a trimmer, a blower, a motor chainsaw, a cut-off machine or the like. The power tool has a clutch and at least one tool member wherein the clutch engages at a coupling engine speed so that the internal combustion engine at engine speeds above the coupling engine speed is operatively connected with the tool member, wherein the internal combustion engine comprises a cylinder in which a piston is reciprocatingly supported and drives a crankshaft that is rotatably supported in the crankcase, wherein the internal combustion engine has an ignition control device that, for each engine speed of the internal combustion engine, specifies at least one ignition timing, wherein the ignition control unit for each engine speed of a first engine speed range specifies a correlated first ignition timing and for each engine speed of a second engine speed range adjoining the first engine speed range specifies a correlated second ignition timing, wherein each first ignition timing is an ignition timing of a first ignition timing range and each second ignition timing is an ignition timing of a second ignition timing range, and wherein each first ignition timing is retarded by at least 5° crank angle relative to each second ignition timing.
U.S. Pat. No. 7,556,020 discloses a method for operating an internal combustion engine of a hand-held power tool wherein the ignition timing is determined based on the engine speed. At idle, ignition timing is retarded. In an acceleration range which adjoins the idle speed range, an advanced ignition timing is adjusted which is advanced further as engine speed increases, except for ignition timing associated with full load operation.
It has been found that in an internal combustion engine which is controlled in this fashion no stable operation in the partial load range is possible. The engine speed fluctuates greatly so that it is impossible for the operator to maintain a constant engine speed. Partial load operation with reduced engine speed relative to full load operation is desirable for specific applications of a hand-held power tool, for example, cutting grass or rice plants. With reduced engine speed, a gentler cut is achieved and subsequent damage to the plants as a result of the cut can be reduced. At the same time, noise emissions are reduced as a result of the reduced engine speed. When operating in partial load range, gyroscopic forces which are caused by rotating masses are lower than at full load so that an improved operating comfort is provided in partial load operation. Moreover, the fuel consumption is reduced compared to full load operation. The rotating masses are in particular the rotating masses of the engine and of the tool member. An internal combustion engine that enables stable operation at partial load is therefore desirable.